1. Field of the Invention
The present invention relates to an ionizing smoke sensor having one radiation source and a two chambered structure comprising of an internal ionization chamber housing an internal electrode having a radiation source and defined by an intermediate electrode, and an external chamber where smoke can flow in, defined by an external electrode and irradiated from the radiation source through an opening in the intermediate electrode.
2. Description of the Related Art
Hitherto, an ionizing smoke sensor having one radiation source and a two chambered structure, for example, the sensor shown in FIG. 8, has been known.
In FIG. 8, an internal ionization chamber 1 (internal chamber) and an external ionization chamber 2 (external chamber) are formed in a body cover 20 and an outer cover 30.
The internal ionization chamber 1 is defined by an insulating member 3 and an intermediate electrode 4 and so constructed that smoke can hardly flow in. An internal electrode 6 having a radiation source 5 which generates pairs of ions is housed in this ionization chamber 1.
The external ionization chamber 2 is defined by an external electrode 7 which covers the outside of the internal ionization chamber 1 and the intermediate electrode 4. A smoke inflow entrance is installed in the external electrode 7 so that smoke can flow in when a fire occurs.
As shown in FIG. 9, an opening 8 is mounted on a part of the intermediate electrode 4 dividing the internal ionization chamber 1 from the external ionization chamber 2 facing the radiation source 5. The radiation irradiated from the radiation source 5 can irradiate the external ionization chamber 2 through this opening 8 and ion pairs can be generated in the external ionization chamber 2.
The operation of this ionizing smoke sensor will be explained as follows.
When voltage is applied between the external electrode 7 and the intermediate 4 and between the intermediate electrode 4 and the internal electrode 6, a faint electric current flows due to the movement of the ion pairs generated by the radiation source 5 from the minus pole to the plus pole due to an electric field generated in each of the ionization chambers and V-I characteristics which generate a specified voltage between the electrodes are obtained.
When the smoke flows into the external ionization chamber 2, the ion electric current flowing between the intermediate electrode 4 and the external electrode 7 decreases and the voltage between the electrodes rises because the mobile speed of the pairs of ions decreases because of the attachment of ion pairs to smoke particles. Then the smoke sensor detects that the smoke concentration has reached a specified value based on the change in the V-I characteristics of the external ionization chamber 2 and sends a fire detecting signal.
However, in this conventional ionizing smoke sensor, since the relatively large opening 8 through which the radiation passes is installed in the intermediate electrode 4 to generate the specified ion pairs in the external ionization chamber 2, a problem occurs, namely, a little amount of thread, moisture, dust and so on which flows into the external ionization chamber 2 invade the internal ionization chamber 1 through the opening 8 and cause misinformation to be given, or a current of air which flows into the internal ionization chamber 1 from the external ionization chamber 2 through the opening 8 carries away the ion pairs, fluctuates the reference voltage and causes sensitivity change to be given.
In addition, since the part where the opening 8 is formed does not have the same function as the intermediate electrode, problems occur, namely, the electric field is hardly formed in the part of the internal ionization chamber 1 facing the opening 8 and hence a sensor having the internal ionization chamber 1 with good V-I characteristics can not be obtained.
Conventional ionizing smoke sensors are described in U.S. Pat. Nos. 4,361,763 and 3,935,492.
Only a bar or a centerpiece is mounted on the opening hole of the intermediate electrode in the smoke sensor of U.S. Pat. No. 4,361,763, and this intermediate electrode is essentially the same as the intermediate electrode shown in FIG. 9 and has problems similar to those of the intermediate electrode shown in FIG. 9.
Although U.S. Pat. No. 3,935,492 discloses a structure having a meshed intermediate electrode, since the whole body of the intermediate electrode is meshed, there are problems in that it is easily affected by a current of air and the electric field is easily fluctuated by ion pairs being carried away.